High-flow cerebral arteriovenous fistulae are well known to present in the neonatal period with macrocephaly, cranial bruits and high-output cardiac failure. This report describes a newborn infant with such a clinical presentation, who had unilateral megalencephaly without macroscopic arteriovenous shunts. Ultrasound Doppler examination of the carotid and cerebral vessels showed diastolic flow, or a decreased pulsatility index, consistent with decreased intracranial vascular resistance. The ipsilateral cerebral arteries and veins were markedly enlarged at angiography and at post-mortem examination, but there was no arteriovenous malformation. The infant expired from high-output cardiac failure and hypoglycemia. It is postulated that the high-output cardiac failure was due to increased blood flow through the enlarged, dysplastic cerebral hemisphere. © 1990 Springer-Verlag.